Kazuma lives in the Lost Ground, an isolated zone created by the Great Uprising. He works as a mercenary, taking various jobs to survive the harsh conditions. His Native Alter ability allows him to break down and reassemble matter, forging powerful combat weapons. This power puts him at odds with HOLY, an organization suppressing Alters. His initial mercenary job involves attacking a plane to rescue a HOLY chairman, primarily using such missions for transportation. After one payment, he gives most of the money to a child whose father was injured, revealing a secretly compassionate side beneath his self-serving exterior. He shares an abandoned dentist's office with Kanami Yuta, hiding his Alter powers from many in the Lost Ground, leading some to see him as lazy or cowardly. However, he deeply cares for those close to him, especially Kanami, whom he treats like a younger sister.

Kazuma's personality blends recklessness with fierce loyalty. He often acts impulsively but processes situations rapidly, displaying a headstrong, rebellious nature that refuses submission to authority. Despite claiming to work solely for payment, he frequently intervenes to protect innocents or loved ones at great personal risk. His unbreakable will prevents him from admitting defeat, evident when he escapes HOLY confinement after capture by Ryuho. This sparks a bitter rivalry with Ryuho, initially viewed as an enemy. Over time, this evolves into mutual respect, though Kazuma remains intensely competitive, constantly seeking rematches to prove his strength. His obsession with defeating Ryuho drives many actions, including entering the Alter Forest to increase his power.

Kazuma's Alter, Shell Bullet, evolves significantly. Its initial form manifests as metallic armor coating his right arm, featuring three curved shoulder fins storing Alter energy. Shattering each fin propels him for enhanced strikes: Shocking First Bullet (smallest fin), Annihilating Second Bullet (middle fin), and Exterminating Last Bullet (largest fin), enabling feats like collapsing buildings and destroying steel gates. Battling a crystalline Alter entity in the Alter Forest, Kazuma tears out its spine, absorbing its energy to evolve Shell Bullet into Shell Bullet Ultimate. This form replaces his right arm entirely with thicker armor, adds facial fins, and incorporates a back-mounted propeller for flight. A hand duct absorbs ambient Alter energy, allowing sustained attacks like Shell Bullet Burst, which disintegrates powerful foes. Later, avenging fallen comrades against Kyoji Mujo, Kazuma pushes beyond his limits to achieve Proud Fist. This final form encases his entire body in armor resembling a lion-samurai hybrid, with a whip-like tail for mobility and dual hand apertures for energy absorption. It grants flight, dimensional tearing capabilities, and explosive punches visible from space. Each evolution strains his body, causing progressive deterioration including a damaged arm and nonfunctional eye.

Key relationships shape Kazuma. Mentorship under HOLY member Straight Cougar provides foundational combat training, particularly in Bullet techniques. The death of non-Alter friend Kunihiko Kimishima, who aids him during a critical battle, fuels his rampage against HOLY and deepens his resolve. After a clash with Ryuho triggers a dimensional upheaval, Kazuma spends eight months in an underground fighting circuit, losing purpose until Mimori Kiryu and Asuka Tachibana help him refocus on protecting Kanami. His protective instincts extend beyond Kanami; he consistently defends vulnerable Lost Ground residents from HOLY or Mainland oppression. This culminates in a temporary alliance with Ryuho to defeat Kyoji Mujo and the Mainland's refined Alter army. Despite cooperation, their rivalry persists, leading to a final battle for supremacy. Years later, Kazuma remains a Lost Ground protector, fending off Mainland incursions while maintaining his rivalry-turned-partnership with Ryuho.

Kazuma's resilience is marked by physical toll and combat feats. His body deteriorates from advanced Alter forms, his arm cracking and eye failing due to energy overload. Notable feats include punching through a crystalline Alter's chest, incinerating a giant robot with one strike, weathering magnitude-8 earthquakes and atmospheric reentry, and generating orbital-scale explosions. He demonstrates exceptional speed, reacting to beams from space and traversing hundreds of kilometers in seconds. His durability lets him withstand lava immersion and country-sized explosions. These capabilities include him among the Lost Ground's most formidable Alter users.